Feedback phaser effect based on a time-varying notch filter with an internal feedback path. The characteristics of the feedback phaser depend on the settings of the time-varying notch filter and on the feedback gain. All quantities are adjustable via the Parameter Window, as summarised in the following table. Stereo signals have separate adjustments for left and right channels. Parameter Purpose "Lower" slider Sets the lower limit for the center-frequency of the sweeping notch (within the allowable range extending from 0 up to the Nyquist frequency (i.e. half the sample rate)). "Upper" slider Sets the upper limit for the center-frequency of the sweeping notch (within the allowable range extending from the lower limit up to the Nyquist frequency). "Rate" slider Sets the rate (in Hz) at which the notch center-frequency sweeps through its profile from the lower limit to the upper limit. The sweep follows an exponential rise from the lower limit to the upper limit, followed by an exponential decay from the upper limit to the lower limit. "Skew" slider Adjusts the time-asymmetry of the shape of the exponential frequency sweep. A value of 0.5 corresponds to a symmetric profile (i.e. the rise profile is a mirror image of the decay profile). "Width" slider Adjusts the bandwidth of the notch as a fraction of the instantaneous center-frequency. As the center-frequency sweeps through its exponential profile, the bandwidth thereby adjusts accordingly. "Depth" slider Adjusts the depth of the notch filter. Maximum depth (lower limit of slider) implies a deep notch; minimum depth (upper limit of slider) implies a shallow notch. The depth is fixed in time. "Feedback" slider Adjusts the internal feedback gain. (CAUTION: values approaching unity can lead to undesirable instabilities.) "Feedback polarity switch" checkbox Toggles the polarity of the internal feedback gain (i.e. switches between negative and positive feedback). "Mix" slider Adjusts the relative amplitudes of the processed ("wet") and raw ("dry") signals. A value of 0.5 corresponds to an equal mix; a value of 1 corresponds to maximum effect; and a value of 0 corresponds to no effect. "Output" slider Adjusts the amplitude of the output signal.
The time-varying notch filter is implemented using a 2-pole IIR all-pass filter connected in parallel with the original signal, thereby creating a notch. The feedback path is connected around the all-pass filter to enhance the effect. See [Zo] for further information on 2-pole filters for audio applications. By implementing the all-pass filter with time-varying coefficients (following an exponential rising and falling profile), the notch center-frequency and bandwidth are also time-varying, as required for creating the phaser effect. The filter coefficient time-history for the exponential variation is stored in a wavetable, and is scanned using a table-lookup oscillator algorithm (see [Moore] chapter 3.1.7 for further information). See [HRM] for general information on audio effects.
